I've found another method to use the sensor by driving the heater with a slow sine. I chose a 285-second period (based on reaching 90% of the steady-state heater temperature) of a 2-volt-to-5-volt PWM'd sine drive. The sensor seems to saturate more easily at lower heater temperatures, so a 3-volt low end - or a smaller load resistance - might reduce that behavior.
The MQ-7 is sold as a CO sensor, but is actually more sensitive to hydrogen than it is to carbon monoxide, according to the documentation. Here is the response using the suggested 5v/1.4v rectangular heater drive. I made two small hydrogen releases of unknown concentration: http://rightime.com/images/Misc/CO_Sens ... ogen_2.GIF
Here is a ~200ppm hydrogen release (25cc into a 30-gallon container) using sine drive: http://rightime.com/images/Misc/MQ-7_5v ... 0kLoad.GIF
The obvious advantage of the sine method is that the sensor output is continuously available using a predictive filter to watch the resulting sine signal, not just single samples at 2.5-minute intervals.
Anyone else working with these sensors?